As someone who runs a lot of Xbow builds, I'm hoping this will help deal with the "disappearing shot" scenario where the animation goes off but the bolt(s) disappear into the great unknown. A few issues I hope you will consider here Steel.
(1) Great Crossbows. I know that "no one" uses them anymore since it's so much easier to get better DPS with a dual-shooter, but if you allow the speed reductions for things like Rapid Reload and Rapid Shot to affect GCs equally, you miss an opportunity to bring this type of crossbow back into parity. Leaving GCs with the live version of these feats while applying the "reduced" version to Dual- and Repeating Xbows seems to me like a net positive. I'm pretty comfortable predicting that no GC build is standing out as a source of "multiple attack" lag.
(2) Imbue builds. Since the advent of the Imbues, there has been a pretty dedicated group of people working to squeeze absolute maximum DPS out of imbue builds. As it stands now, throwers (particularly shuriken throwers) already have an advantage in the imbue builds since a single throw animation can produce multiple attacks, each with chances to have doubleshot procs. As EinarMal has noted above, if you don't balance doubleshot %s you are just going to push Xbow builds into thrower builds... not fixing the problem you note.
(3) Balancing everything out. Related to the imbue issues noted above, there is already a slight advantage to using light crossbows vs their heavy counterparts due to the attack speed if you aren't already hitting the animation cap. If you reduce the rate of fire like you are attempting here, you are going to see a bunch of people swap over to light crossbows to get back up to the max animation speed... and this is likely to defeat your goals here. To keep people using the heavy versions, I would suggest changing the damage die slightly to make up for this distance. Just looking at the most common Heavy Crossbow in epic leveling (Storm), you could change that from 3(d12)+6 to 3(2d6)+6. This will keep many people using Storm instead of swapping over to the Diplomancer (3[d8]+6) for the increase in rate of fire.